There are many known methods and devices for coating a moving web and other fixed or moving substrates, and for smoothing the resulting coating. Several are described in Booth, G. L., “The Coating Machine”, Pulp and Paper Manufacture, Vol. 8, Coating, Converting and Processes, pp 76-87 (Third Edition, 1990) and in Booth, G. L., Evolution of Coating, Vol. 1 (Gorham International Inc.). For example, gravure roll coaters (see, e.g. U.S. Pat. No. 5,620,514) can provide relatively thin coatings at relatively high run rates. Attainment of a desired specific average caliper usually requires several trials with gravure rolls of different patterns. Runtime factors such as variations in doctor blade pressure, coating speed, temperature, or liquid viscosity can cause overall coating weight variation and uneven localized caliper in the machine or transverse directions.
Barmarks and chatter marks are bands of light or heavy coating extending across the web. These are regarded as defects, and can be caused by factors such as vibration, flow pulsation, web speed oscillation, gap variation and roll drive oscillation. Chatter marks are commonly repeating, but barmarks can occur as the result of random system upsets. Gutoff and Cohen, Coating and Drying Defects (John Wiley & Sons, New York, 1995) discusses many of the sources of cross web marks and emphasizes their removal by identifying and eliminating the fundamental cause. This approach can require substantial time and effort.
Under some gravure roll coating run conditions, a gravure roll pattern appears in the wet coating. Gravure roll marks can be removed with an arcuate flexible smoothing film located down web from the gravure roll (see, e.g., U.S. Pat. No. 5,447,747); with a smoothing roll or rolls bearing against an intermediate coating roll (see, e.g., U.S. Pat. No. 4,378,390) or with a set of smoothing rolls located down web from the gravure roll (see, e.g., U.S. Pat. No. 4,267,215).
Very thin coatings (e.g., about 0.1 to about 5 micrometers) can be obtained on gravure roll coaters by diluting the coating formulation with a solvent. Solvents are objectionable for health, safety, environmental and cost reasons.
Multiroll coaters (see, e.g., U.S. Pat. Nos. 2,105,488; 2,105,981; 3,018,757; 4569,864 and 5,536,314) can also be used to provide thin coatings. Multiroll coaters are shown by Booth and are reviewed in Benjamin, D. F., Anderson, T. J. and Scriven, L. E. “Multiple Roll Systems: Steady-State Operation”, AIChE J., V41, p. 1045 (1995); and Benjamin, D. F., Anderson, T. J. and Scriven, L. E., “Multiple Roll Systems: Residence Times and Dynamic Response”, AIChE J., V41, p. 2198 (1995). Commercially available forward-roll transfer coaters typically use a series of three to seven counter rotating rolls to transfer a coating liquid from a reservoir to a web via the rolls. These coaters can apply silicone release liner coatings at wet coating thickness as thin as about 0.5 to about 2 micrometers. The desired coating caliper and quality are obtained by artfully setting roll gaps, roll speed ratios and nipping pressures. Another type of coating device that could be described as a multiroll coater is shown in U.S. Pat. No. 4,569,864, which describes a coating device in which a thick, continuous premetered coating is applied by an extrusion nozzle to a first rotating roll and then transferred by one or more additional rolls to a faster moving web.